These days it is not enough to have a website, oh no, those days are long gone. Even for business geared apps, it is expected not only good online presence, but also apps for major phones and tablets. This obviously puts a lot of pressure on founders of new companies as it requires effort times 3, often more to get to same place as before.
Some popular startup thinkers suggested mobile first strategy for several reasons. First one would be that you get into hands of potential customers, target audience quickly, so if you have to develop something, do it mobile first, that way you will always have that essential mobile presence and you can always develop website later.
For a large number of startups this makes sense.
Another argument in favor of this approach is that mobile first also provides best experience on mobile devices, as it was developed for them first, not as a copy of the website.
Another advantage with this approach is, that is very hard finding mobile developers who are good, so once you are done with that, other things are much easier.
In essence, mobile experience is very unique format and developing specifically for it does bring better experience for end user whoever that might be. Also, features like location, position, snapping pictures, these are all things that are natural for phone or tablet and not so much for desktop.
Main dissadvantage is that still most activity is happening on the web a.k.a. desktop and not having a website can put you at disadvantage when you need to be discovered. It is much harder making people install new app on their already overcrowded phones and tablets.
If you app is genuinely unique, it would be very hard to explain that without good website.
Another really worrying trend for all the mobile apps is that people don’t stick with them, they use it for a short while and then abandon for next thing. It has to be really super useful to stay on the phone or tablet.
So far middle ground is to develop webapp with responsive design. Responsive design even with frameworks that aid it’s development is hard to do right, because it is simply one site to serve two different platforms.
For some apps, this will work well and it will be just fine. For some, not so much.
Also, for games, making it mobile first will make it very addictive as customers can play it at any moment in time. On the other hand, making it like facebook app would allow this app to be spread virally much easily. FB apps can be mobile as well, but it is much easier to notice others playing and be inticed into playing if you are on the desktop.
I personally like responsive design that adjusts to mobile, tablet or desktop. It allows me to orchestrate user experience from single point. There are apps that simply require well done mobile app.